It is known that, in lithography methods, imaging errors can occur if the structures to be imaged become very small and have a critical size or a critical distance with respect to one another. The critical size is generally referred to as the “CD” value (CD: critical dimension).
What is more, imaging errors may occur if structures are arranged so closely next to one another that they mutually influence one another during the imaging. These imaging errors based on “proximity effects” can be reduced by modifying the mask layout beforehand with regard to the “proximity phenomena” that occur. Methods for modifying the mask layout with regard to avoiding proximity effects are referred to by experts by the term OPC methods (OPC: optical proximity correction).
FIG. 1 illustrates a lithography process without OPC correction. The illustration reveals a mask 10 with a mask layout 20 that is intended to produce a desired photoresist structure 25 on a wafer 30. The mask layout 20 and the desired photoresist structure 25 are identical in the example in accordance with FIG. 1. A light beam 40 passes through the mask 10 and also a focusing lens 50 arranged downstream and falls onto the wafer 30, thereby imaging the mask layout 20 on the wafer 30 coated with photoresist. On account of proximity effects, imaging errors occur in the region of closely adjacent mask structures with the consequence that the resulting photoresist structure 60 on the wafer 30 in part deviates considerably from the mask layout 20 and thus from the desired photoresist structure 25. The photoresist structure that results on the wafer 30, the photoresist structure being designated by the reference symbol 60, is illustrated in enlarged fashion and schematically beneath the wafer 30 for improved illustration in FIGS. 1 and 2.
In order to avoid or to reduce these imaging errors, it is known to use OPC methods that modify the mask layout 20 beforehand in such a way that the resulting photoresist structure 60 on the wafer 30 corresponds to the greatest possible extent to the desired photoresist structure 25.
FIG. 2 shows a previously known OPC method described in the document “A little light magic” (Frank Schellenberg, IEEE Spectrum, September 2003, pages 34 to 39), which is incorporated herein by reference, in which the mask layout 20′ is altered compared with the original mask layout 20 in accordance with FIG. 1. The modified mask layout 20′ has structure alterations that are smaller than the optical resolution limit and, therefore, cannot be imaged “1:1”. These structure alterations nevertheless influence the imaging behavior of the mask, as can be discerned at the bottom of FIG. 2. This is because the resulting photoresist structure 60 corresponds distinctly better to the desired photoresist structure 25 than is the case with the mask in accordance with FIG. 1.
In the case of the previously known OPC methods by which a “final” mask layout (see mask 20′ in accordance with FIG. 2) is formed from a provisional auxiliary mask layout (e.g., the mask layout 20 in accordance with FIG. 1), a distinction is made between so-called “rule-based” and “model-based” OPC methods.
In the case of rule-based OPC methods, the formation of the final mask layout is carried out using rules, in particular tables, defined beforehand. The method disclosed in the two U.S. Pat. Nos. 5,821,014 and 5,242,770 (both of which are incorporated herein by reference), by way of example, may be interpreted as a rule-based OPC method, in the case of which optically non-resolvable auxiliary structures are added to the mask layout according to predetermined fixed rules, in order to achieve a better adaptation of the resulting photoresist structure (reference symbol 60 in accordance with FIGS. 1 and 2) to the desired photoresist structure (reference symbols 25 in accordance with FIGS. 1 and 2). In the case of these methods, then, a mask optimization is carried out according to fixed rules.
In model-based OPC methods, a lithography simulation method is carried out, in the course of which the exposure operation is simulated. The simulated resulting photoresist structure is compared with the desired photoresist structure, and the mask layout is varied or modified iteratively until a “final” mask layout is present, which achieves an optimum correspondence between the simulated photoresist structure and the desired photoresist structure. The lithography simulation is carried out with the aid of, for example, a DP-based lithography simulator that is based on a simulation model for the lithography process. For this purpose, the simulation model is determined beforehand by “fitting” or adapting model parameters to experimental data. The model parameters may be determined for example by evaluation of so-called OPC curves for various CD values or structure types. One example of an OPC curve is shown in FIG. 6 and will be explained in connection with the associated description of the figures. Model-based OPC simulators or OPC simulation programs are commercially available. A description is given of model-based OPC methods for example in the article “Simulation-based proximity correction in high-volume DRAM production” (Werner Fischer, Ines Anke, Giorgio Schweeger, Jörg Thiele; Optical Microlithography VIII, Christopher J. Progler, Editor, Proceedings of SPIE VOL. 4000 (2000), pages 1002 to 1009) and in the German Patent No. DE 101 33 127 C2, both of which are incorporated herein by reference.
Irrespective of whether an OPC method is a model-based or a rule-based OPC method, OPC variants can also differ with regard to their respective optimization aim. By way of example, so-called “target” OPC methods and so-called process window OPC methods, for example “defocus” OPC methods, have different optimization aims.
The aim of target OPC methods is to hit as accurately as possible the predefined target for the individual geometrical dimensions of the mask structures in the case of correctly complying with all the predefined technological and method conditions (e.g., focus, exposure dose, etc.). Thus, in the case of a target OPC variant it is assumed that all the predefined process parameters are “hit” or set and complied with in an ideal way. In this case, the term “target” is understood to mean the structure size of the main structures to be imaged.
Since the gate length of transistors is of crucial importance for their electrical behavior, target OPC methods are used in particular for the gate plane of masks. What is disadvantageous in the case of the target OPC variant, however, is that the predefined geometrical dimensions of the mask structures are actually complied with only when the predefined process parameters are complied with in a quasi exact fashion. If fluctuations in the process parameters occur, it is possible for, in some instances, considerable deviations to occur between the desired mask structures or mask dimensions and the actual resulting mask structures or mask dimensions. This may lead, for example, to a tearing away of lines or to a short circuit between lines. The resulting process window is, therefore, generally relatively small in the case of a target OPC method.
By contrast, process window OPC methods, for example defocus OPC methods, have the aim of making the process window—that is to say the permissible parameter range of the process parameters for the exposure process with the resulting mask—as large as possible in order to ensure that the mask specifications are complied with even in the case of process fluctuations. In this case, with defocus OPC methods it is accepted that the geometrical mask target dimensions are not met exactly. Deviations are, therefore, deliberately accepted in order to enlarge the process window and thus the tolerance range during later use of the mask.
A defocus OPC method is described for example in the above-mentioned German Patent No. DE 101 33 127. This method involves predefining a “fictitious” defocus value, which is taken as a basis for the simulation of the exposure operation. This defocus value specifies that the resist structure to be exposed with the mask lies somewhat outside the optimum focal plane. In the context of the OPC method, an attempt is made to achieve an optimum imagining behavior of the mask despite the defocusing purportedly present. Thus, an attempt is made to compensate for the imaging error caused by the purported defocusing. This “compensation operation” has the effect of changing the form of the mask layout in such a way that the line structures are made wider and as well a larger distance is produced between two adjacent line structures in each case. As a result, a mask is thus obtained with which, when using a focused exposure, the probability of the formation of wider line structures and the formation of larger distances between respectively adjacent line structures is greater than the probability of the formation of excessively small line structures and the formation of excessively small distances between adjacent line structures.